With the increasing popularity of wide area networks, such as the Internet and/or the World Wide Web, network growth and traffic have exploded in recent years. Network users continue to demand faster networks, and as network demands continue to increase, existing network infrastructures and technologies are reaching their limits.
An alternative to existing hardwire or fiber network solutions, which suffer from limited capacity or exponentially increasing construction costs in “the last mile” of the communication system, is the use of wireless optical telecommunications technology. Wireless optical telecommunications utilize beams of light, such as lasers, as optical communication signals, and therefore do not require the routing of cables or fibers between locations. Data, or other information, is encoded into a beam of light, and then transmitted through free space from a transmitter to a receiver.
For point-to-point free-space laser communications, the use of narrow optical beams provides several advantages, including data security, high customer density, and high directivity. High directivity makes the achievement of high data rates and high link availability easier, due to higher signal levels at a receiver. In order to take full advantage of this directivity, some form of tracking is often necessary to keep the antennas of a transmitter and of the receiver properly pointed at one another. For example, a transmitted optical beam with a 1-mrad divergence has a spot diameter at the receiver of about 1 m at a 1-km range. Thus, movement of the transmitter or receiver by even a small fraction of the divergence (or field of view) could compromise the link unless some form of active tracking is employed.
Charge coupled device (“CCD”) arrays or quadrant cell optical detectors (hereinafter referred to as “quad cells,” or “quad cell detectors”) may be used as tracking detectors in a tracking system. In either case, an electrically controllable steering mirror, gimbal, or other steering device may be used to maximize an optical signal (e.g., light) directed at a high speed detector, based on information provided by the tracking detector. This is possible since optical paths for tracking and communication are pre-aligned, and the nature of a tracking signal for a perfectly aligned signal is known. CCD tracking is very sensitive, offers potentially more immunity to solar glint because of the ability to ignore glint “features” on the CCD array, and is in general, a well-proven tracking method. However, at certain wavelengths, a lower wavelength tracking beam is often necessary due to limitations of CCD detection systems. Such separate wavelengths are typically used with their own set of transmitter optics, thereby requiring the use of additional hardware. Furthermore, designs using separate beacon and communication optical transmitters require more time in manufacturing because of the need to co-align the two optical transmitters. Such separate transmitter paths are also more susceptible to misalignments due to mechanical shock and/or thermal stresses.
In the case of quad cells, a majority of the received optical signal is typically directed to the high-speed detector for the communication channel, while a small portion (e.g., 10 percent) is split off or directed to the tracking detector. For an aligned optical system, an equal signal in all four quadrants will normally indicate that the steering mirror has optimally directed the optical communication signal onto the high speed detector, and where there is deviation from this alignment, the steering mirror will direct the optical signal back to this optimum equilibrium.
One method of signal detection via a quad cell utilizes a low frequency tone superimposed on a data communication signal which can be recovered using a variety of methods in the receive electronics. An example of such a method is described in detail in commonly assigned U.S. patent application Ser. No. 09/627,819, entitled METHOD AND APPARATUS FOR TONE TRACKING IN WIRELESS OPTICAL COMMUNICATION SYSTEMS, filed Jul. 28, 2000. This method uses a tone (e.g., 20 kHz) superimposed on a data communication signal and having a small modulation depth as compared with the primary digital or modulated data communication signal. The modulation depth of the 20 kHz tone may be as little as a few percent of the amplitude of an on-off keying (“OOK”) signal used to convey digital information, so as not to adversely impact the data communication channel. The advantage of tone modulation detection is an enhanced sensitivity gained via use of a narrow-band electronic filter or lock-in detector that will eliminate wide-band electronic noise.
As an alternative to the methods described in the aforementioned commonly assigned application, or to aid in the system level pointing and tracking of a free-space optical communication link, auxiliary communication channels between the transmitter and the receiver are also advantageous. Communication of auxiliary system level information between terminals of a free-space optical network facilitates effective signal transmission by providing link status information, transmit power control information, and alignment information, including pointing, acquisition, and tracking algorithms. This auxiliary information, in one form or another, may be essential to maintaining an efficient communication link between two free-space optical terminals. In particular, the communication of power control information, based on current signal reception, will increase communication efficiency and data rates by indicating whether the strength of the received signal needs to be optimized. Similarly, the communication of auxiliary alignment information may provide better tracking coordination (e.g., using a master/slave control system), and facilitate the exchange of other system level information that is useful for the reliable operation of the free-space optical communication link.
As will be apparent to the reader, use of the primary communication channel of the free-space optical link to transmit auxiliary system communications has the disadvantage of requiring that the pointing and tracking system already be working before the primary communication channel can be used in this manner (the primary use of the auxiliary communication channel may be to assist the pointing and tracking system in order to establish a reliable communication link). Other possible auxiliary communication channels include modems, Internet links, or a radio frequency (“RF”) channel. However, each of these auxiliary communication channels also contain inherent disadvantages. Use of a modem requires a telephone line, RF adds complexity and cost to the system, and an Internet connection requires that an additional back-up network be in place. As such, methods of transmitting auxiliary communications between terminals of a free-space optical communication system that can resolve the aforementioned difficulties are needed.